This invention relates to reverse osmosis membranes, their preparation, and their use in processes for desalination of water.
Reverse osmosis is an established process for removing various dissolved salts from water. In such a process, a pressure which exceeds the osmotic pressure of the feed solution is applied to a saline water feed solution separated from purified water by a semipermeable membrane. As a result, water diffuses through the membrane with most salt molecules being rejected by the membrane to remain in the saline feed solution.
While desalination by reverse osmosis is known, certain drawbacks attending practice of the process using known equipment and materials have tended to limit its acceptance. For example, when practicing reverse osmosis utilizing a sheet-like membrane to separate the feed solution from the purified water, the sheet configuration of the membrane severely limits the available membrane surface area. It is recognizd that a membrane may be formed as a bundle of hollow fibers, this being effective to substantially increase the membrane surface area. However, for a number of reasons, the use of such hollow fibers in reverse osmosis desaliniation has not been entirely successful.
A hollow fiber bundle ordinarily is utilized in reverse osmosis desalination with the feed directed against the shell or outer sides of the fibers. This is done because a hollow fiber, subjected to a difference in fluid pressure on opposite sides of the wall of the fiber, is better able to withstand the pressure differential without rupturing when the elevated pressure is exerted on the outside of the fiber to exert radially inwardly directed forces, rather than on the inside of the fiber to exert radially outwardly directed forces. However, concentration polarization and fouling are severe problems in systems which rely on such shell side feed. The usual brackish water includes, in addition to dissolved salt, suspended solid particulate material. With shell side feed systems utilizing relatively low flow rates of feed solution, stagnant areas or "dead" spots within the fiber bundle result in collection of such material on the membrane surface to reduce its efficiency. In addition, salt concentration tends to build up in stagnant areas, thereby producing concentration polarization which further decreases efficiency.
Tubes have been suggested for use in membrane systems with the feed solution being pumped through the lumens or bores of the tubes. For instance, microporous polyvinyl chloride tubes have been proposed as the support for a reverse osmosis membrane. These tubes are prepared by melt extrusion of a mixture of polyvinyl chloride and micronized sodium chloride. After cooling, the sodium chloride is leached from the support to form the pores in the structure. Such tubes have a high burst pressure and are good supports for cellulose acetate membranes prepared by the Loeb-Sourirajan technique. However, a cellulose acetate membrane so prepared is characterized by a low flux or flow rate and inferior rejection of salt. Moreover, these tubes are too expensive to be widely used.
In NTIS Report No. PB-248 666 (1975), Cabasso et al. studied the development of outside-skinned coated hollow polysulfone fibers for reverse osmosis desalination of seawater, disclosing hollow fiber spinning of polysulfone with various additives, including methyl Cellosolve.RTM. (MeC), fatty acids, PVP, and polyethylene glycol (PEG). Satisfactory spinning solutions were prepared containing 10% PVP, 10% MeC and 10% PEG, the PEG fibers failing under pressure applied to the outside of the fibers. However, no consideration was given to or determination made as to coating with an ultrathin solute barrier membrane on the inside or lumens of the fiber.
U.S. Pat. No. 4,051,300 to Klein et al. discloses porous hollow fibers for use in separation schemes such as reverse osmosis, ultrafiltration and gas separations. These fibers are said to be designed so that ultrathin solute barrier coatings can be supported on either their external or internal surfaces, and so that a solvent containing dissolved solute, under pressure, will first contact the ultrathin solute barrier coating, with the solvent then passing through the hollow porous fiber, leaving the solute behind. The porous support fibers disclosed in the Klein et al. patent are prepared by hollow fiber spinning an organic solution containing a fiber-forming polymer (polysulfones and aromatic polyamide polymers are disclosed) together with polyvinylpyrrolidone (PVP) as a pore- forming material. However, such supports do not possess the proper combination of the properties of sufficient burst strength, water permeability and coatability to meet the requirements of successful lumen-side feed reverse osmosis operation.